Major injury and its sequelae cause significant immune dysfunction. A breakdown in immune regulation leads to increased morbidity and mortality due to sepsis, shock, systemic inflammatory response syndrome, and multiple organ dysfunction syndrome. Understanding the fundamental mechanisms underlying this process and the mechanisms by which host defenses fail to respond appropriately to infectious challenge is critical to reduce injury-associated mortality. Mononuclear phagocytes act directly to phagocytose and kill microbes, and indirectly through antigen presentation, cell-cell signaling, and cytokine production. Aims achieved through the previous grant cycle have characterized global defects in macrophage phagocytosis, cytokine production and antigen presentation as well as defects in afferent and efferent signaling pathways. These studies document a time-dependent cellular hypoactivity followed by a hyperactive response to cell stimulation, which is associated with significantly decreased survival to varied septic challenges 7 days after injury. Finally, we have shown that the macrophage hyperactive state is associated with upregulation of COX-2 and iNOS, and secretion of the immune regulatory agents PGE2 and NO , which are likely to have substantial impact on subsequent immune function. These late changes in macrophage function following injury require further investigation to elucidate the underlying mechanisms and develop strategies for improving outcome. The major hypothesis within this proposal is that increased expression of COX-2 and iNOS is crucial to the development of the delayed immune dysfunction seen after injury. In this revised application we will investigate the role of COX-2 and iNOS upregulation on the development of a late hyperactive macrophage state. Furthermore, we propose to evaluate and characterize the regulatory mechanisms contributing to macrophage hyperactive responses to injury by assessing intracellular pathways and neuroendocrine responses regulating macrophage COX-2 and iNOS function. The role of COX-2 and its relation to iNOS in regulating macrophage function in injury will be assessed in vivo using knockout mice. Lastly, we will determine the effects of a selective COX-2 inhibitor on immune function in trauma patients, in a randomized, prospective, double blind clinical study. These experiments will elucidate the role of prostaglandins and NO in late macrophage dysfunction after injury, and will provide valuable information for the development of novel strategies for the treatment of patients after injury.